i) Field of the Invention
The present invention relates to a method of controlling the dispersibility and barrier properties of dried forms of nanocrystalline cellulose (NCC) in aqueous media by controlling the ionic strength and/or the pH of the aqueous media.
ii) Description of the Prior Art
Cellulose whiskers or nanocrystals are obtainable by controlled acid hydrolysis of cellulose from various cellulose sources, in particular from wood pulp and cotton. The less-dense amorphous regions along the cellulose microfibril are more susceptible to acid attack during hydrolysis and cleave to give cellulose nanocrystals [1,2]. Their low cost, renewability and recyclability, make nanocrystalline cellulose whiskers attractive for various applications [3,4] as does their chemical reactivity which allows the tailoring of their chemical and physical properties.
NCC is a renewable, recyclable, carbon neutral material. These factors and the potentially unique mechanical and optical properties of NCC have generated great interest in manufacturing NCC-based products at an industrial scale. However, because NCC is initially produced as an aqueous suspension with only a few weight percent solids content, any high-volume application will require NCC to be delivered in dried form and resuspended at the site of use in order to minimise both cost and shipment size and weight.
Evaporation of aqueous NCC suspensions produces solid semi-translucent NCC films that retain the chiral nematic liquid crystalline order inherent to NCC suspensions above a critical concentration. These films can be produced at basis weights ranging from approximately 40-175 g/m2. These films because of ordered self-assembly are physically strong, have barrier properties and can be made to reflect circularly polarized light in a narrow wavelength band. Freeze-drying of NCC suspensions produces a product with a texture ranging from flaky lamellar to a solid foam to a soft powder.
Nanocrystalline cellulose suspensions produced by sulfuric acid hydrolysis are not dispersible in water or other liquid media once they have been fully dried to solid films [5]. The proton counterions can be exchanged for other neutral monovalent counterions M+ such as Na+, K+, Cs+, NH4+, and tetraalkylammonium and trialkylamine ions by titrating with the appropriate M+ hydroxide; when dried, these forms of NCC have been found to be completely redispersible in water [6].